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You open the closet and test the appliance with the door open. State in your notes that worst case was reached with the appliance door closed (make sure that is the case, often the greatest depressurization is with the door to the CAZ open)

You can stand outside and read pressures. Be sure to have extra hose as you need CAZ wrt outside pressure taken on the A channel, run a hose under the closet door (Input) and another hose to outside. Also you'll need vent wrt CAZ taken on the B channel , run a hose under the closet door into the vent (input) and another hose under the closet door (reference). Set up in worst case, fire the appliance, shut the door and monitor the pressure, if the vent is positive wrt CAZ the appliance is backdrafting.

For water heaters we also use a combustible gas leak detector on top of the water heater near the draft hood if it sounds off theres spillage.